Thick film materials are mixtures of metal, glass and/or ceramic powders dispersed in an organic vehicle. These materials are applied to nonconductive substrates to form conductive, resistive or insulating films. Thick film materials are used in a wide variety of electronic and light electrical components.
The properties of individual compositions depend on the specific constituents which comprise the compositions. All compositions contain three major components. The conductive phase determines the electrical properties and influences the mechanical properties of the final film. In conductor compositions, the conductive phase is generally a precious metal or mixture of precious metals. In resistor compositions the conductive phase is generally a metallic oxide. In dielectric compositions, the functional phase is generally a glass or ceramic.
The binder is usually a glass, a crystalline oxide or a combination of the two. The binder holds the film together and to the substrate. The binder also influences the mechanical properties of the final film.
The vehicle is a solution of polymers in organic solvents. The vehicle determines the application characteristics of the composition.
In the composition, the functional phase and binder are generally in powder form and have been thoroughly dispersed in the vehicle.
Thick film materials are applied to a substrate. The substrate serves as a support for the final film and may also have an electrical function, such as a capacitor dielectic. Substrate materials are generally nonconducting.
The most common substrate materials are ceramics. High-purity (generally 96%) aluminum oxide is the most widely used. For special applications, various titanate ceramics, mica, beryllium oxide and other substrates are used. These are generally used because of specific electrical or mechanical properties required for the application.
In some applications where the substrate must be transparent--such as displays--glass is used.
Thick film technology is defined as much by the processes as by the materials or applications. The basic thick film process steps are screen printing, drying and firing. The thick film composition is generally applied to the substrate by screen printing. Dipping, banding, brushing or spraying are occasionally used with irregular shaped substrates.
The screen printing process consists of forcing the thick film composition through a stencil screen onto the substrate with a squeegee. The open pattern in the stencil screen defines the pattern which will be printed onto the substrate.
After printing, the film is dried and fired--generally in air at a peak temperature of 500.degree.-1000.degree. C. This process forms a hard, adherent film with the desired electrical and mechanical properties.
Additional thick film compositions may be applied to the same substrate by repeating the screen printing, drying and firing processes. In this way, complex, inter-connected conductive, resistive and insulating films can be generated.
Thick film resistor compositions are usually produced in decade resistance values and materials are available that provide a wide range of sheet resistance (0.5 .OMEGA./.quadrature. to 1.times.10.sup.9 .OMEGA./.quadrature.). A change in length to width aspect ratio of a resistor will provide resistance values lower than 0.5 .OMEGA./.quadrature. and higher than 1.times.10.sup.9 .OMEGA./.quadrature. and any intermediate resistance value.
Composition blending is a technique widely used to obtain resistance value between standard decade values. Adjacent decade members can be mixed in all proportions to produce intermediate values of sheet resistance. The mixing procedure is simple but requires care and the proper equipment. Usually blending has minimal effect on Temperature Coefficient of Resistance.
High stability and low process sensitivity are critical requirements for thick film resistor compositions for microcircuit applications. In particular it is necessary that resistivity (R) of the films be stable over a wide range of temperature conditions. Thus, the Thermal Coefficient of Resistance (TCR) is a critical variable in any thick film resistor composition. Because thick film resistor compositions are comprised of a functional or conductive phase and a permanent binder phase, the properties of the conductive and binder phases and their interactions with each other and with the substrate affect both resistivity and TCR.
Functional phases based on ruthenium chemistry form the core of conventional thick film resistor compositions.
Ruthenium compounds based on the pyrochlore family have a cubic structure with each ruthenium atom surrounded by six oxygen atoms, forming an octahedron. Each oxygen atom is shared by one other octahedron to form a three-dimensional network of Ru.sub.2 O.sub.6 stoichiometry. The open areas within this framework are occupied by large cations and additional anions. A wide range of substitution in this secondary lattice is possible which makes for a great deal of chemical flexibility. The pyrochlore structure with the general formula A.sub.2 B.sub.2 O.sub.6-7 is such a flexible structure. Pyrochlores which behave as metals, semiconductors or insulators can be obtained through controlled substitution on available crystallographic sites. Many current pyrochlore based thick film resistors contain Bi.sub.2 Ru.sub.2 O.sub.7 as the functional phase.
Ruthenium dioxide is also used as the conductive phase in thick film resistor compositions. Its rutile crystal structure is similar to that of pyrochlore in that each ruthenium atom is surrounded by six equidistant oxygen atoms forming an octohedron. However, in the rutile structure each oxygen is shared by 3 octahedra. This results in a complex three-dimensional network in which, in contrast to the case of pyrochlore, chemical substitution is very limited.
In the formulation of thick film resistor compositions for particular applications, it is often found that the TCR for the anticipated temperature range in use is too high and it therefore becomes necessary to increase or reduce the TCR in order that the resistivity not change too much over the operating range of temperature. It is well known in the thick film resistor art that additions of small amounts of various inorganic compounds will accomplish this. For example, in ruthenium-based resistors it is known to employ for this purpose CdO, Nb.sub.2 o.sub.5, TiO.sub.2, MnO.sub.2, Mn.sub.2 O.sub.3, V.sub.2 O.sub.5, NiO, Sb.sub.2 O.sub.3 and Sb.sub.2 O.sub.5, all of which are negative TCR "drivers". That is, they reduce TCR. On the other hand, CuO is known as a positive TCR drive in ruthenium-based resistors.
In the usual formulation of resistors, it is found that negative TCR drivers lower TCR, but simultaneously raise resistivity (R). Conversely, positive TCR drivers raise TCR but lower resistivity.
A recurrent problem with the use of the prior art materials used as negative TCR drivers is that the resistivity of the resistors in which they are used is raised excessively when the desired level of TCR reduction is obtained. This is a disadvantage because it necessitates the inclusion of additional conductive phase metals to obtain the same resistivity level. In turn, the inclusion of additional conductive phase adversely affects the resistance stability of the fired resistor with respect to time.